This invention relates to steel wires formed with an alloy coating and a method for producing the same, particularly relates to an iron-zinc-aluminum alloy coated spring steel wires and method for producing the same.
Steel wires for springs have been required, except for valve springs for use in an automotive vehicle, to principally have:
(1) High formability, and
(2) High corrosion resistance.
Conventionally, the following spring steel wires have been practically used.
AISI304 STAINLESS STEEL WIRE FOR SPRING: This wire is produced by drawing a AISI304 wire.
ZINC-PLATED STEEL WIRE FOR SPRING: This wire is produced by plating a high carbon spring steel wire or piano wire with zinc and drawing the zinc-plated steel wire, or alternatively drawing a high carbon spring steel wire and plating the drawn spring steel wire with zinc.
IRON-ZINC ALLOY COATED STEEL WIRE FOR SPRING: This wire is produced by forming a steel wire with an iron-zinc alloy coating. The production of this wire is described in Japanese Examined Patent Publication No. 55-37590.
HIGH CARBON STEEL WIRE FOR SPRING: This wire is also called PIANO or MUSIC WIRE, and is widely used for springs. This wire has 0.60 to 0.95 weight percent carbon and a high tensile strength. According to JIS (Japanese Industrial Standards), there are provided ten or more classes for high carbon spring steel wires in the above-mentioned range of carbon content. Besides carbon, this wire contains 0.12 to 0.32 weight percent silicon, 0.30 to 0.90 weight percent manganese, and a negligible amount of phosphorus, sulfur, copper and the like.
These spring steel wires have the following disadvantages and do not satisfy completely the aforementioned Requirements (1) and (2), i.e., the formability and corrosion resistance.
AISI304 STAINLESS STEEL WIRE FOR SPRING: This wire is excellent in corrosion resistance, but poor in formability in that there are variations in length of formed coil springs. Accordingly, this spring wire does not satisfy Requirement (1).
ZINC-PLATED STEEL WIRE FOR SPRING: This wire is covered with a thick and soft zinc layer, which is likely to gall in spring forming, e.g., when forming into a coil spring. Accordingly, this wire has poor formability and thus is unsatisfactory for Requirement (1). In the aspect of corrosion resistance, this wire has relatively good resistance for red rust, but has poor resistance for white rust. This wire gathers white rust at an early stage. Thus, it cannot be said that this wire satisfies Requirement (2).
IRON-ZINC ALLOY COATED STEEL WIRE FOR SPRING: This wire is covered with an iron-zinc alloy coating, which reduces the friction coefficient between a machine tool and a surface of steel wire for spring when being formed into springs. Accordingly, this wire has an excellent formability and satisfies Requirement (1). However, this wire is plated with zinc to form the iron-zinc alloy coating on the surface thereof. The iron-zinc alloy coated wire is then drawn. In the drawing, cracking is likely to occur in the alloy coating, resulting in partial peel-off of the alloy coating. Thus, this wire has a poor corrosion resistance and does not satisfy Requirement (2).
HIGH CARBON STEEL WIRE FOR SPRING: This wire sufficiently retains lubricant used in spring forming on the surface thereof, and can accordingly maintain a required formability. However, this wire has a poor corrosion resistance since no metal coating is formed on the surface thereof, and therefore does not satisfy Requirement (2).
As described above, each one of the conventional spring steel wires has merits and demerits. No such steel wire has been available which satisfies both Requirement (1) of good formability and Requirement (2) of good corrosion resistance.
Further, a hot-dipped zinc-aluminum plated wire has been known which has an iron-zinc-aluminum alloy layer and zinc-aluminum alloy plating on the alloy layer. This wire has been used for normal use, such as chain link wire net for cultivating fish in the sea, core for aluminum cable steel reinforced, but not used for springs because of not satisfying Requirements (1) and (2).